This disclosure relates to a method for closed-loop control of the surface temperature of any glow plug from a specific series in an internal combustion engine, using a glow plug control device that acts on the glow plug connected thereto with a pulse-width-modulated effective voltage and in which a temperature model displaying the behaviour of the series is stored.
A method of this type is known from German Publication No. DE 10 2006 060 632, in which the temperature model is fed with parameters of the glow plug and other operating variables. A model temperature is established in accordance with these input variables and corresponds to the surface temperature of the glow plug. A target resistance for the glow plug is established from the deviation of the model temperature from a target temperature and the current resistance of the glow plug is controlled to the target resistance by a control system.
German Publication No. DE 10 2008 040 971 A1 describes a method of the type mentioned in the introduction in order to correct a base control of the glow plug, which is carried out with an effective voltage established from a characteristic map. The temperature model calculates a model temperature of the glow plug from the resistance measured at the glow plug. This model temperature is then compared with the target temperature. The effective voltage established from the characteristic map is adapted accordingly on the basis of the deviation.
A closed-loop control method is known from U.S. Publication No. 2011/0220073, in which the control is likewise based on the assignment of a temperature to an electrical resistance. To improve the control it is proposed to measure the combustion chamber pressure using a pressure sensor of the glow plug and to use this for correction of the resistance expected for the target value of the surface temperature of the glow plug in order to take into account approximately the cooling or heating effect of combustion gases.
The quality of the temperature control achieved with the known methods is poor, however. This is true in particular for ceramic glow plugs, with which there are strong variations of the cold resistance as a result of the manufacturing process. An unambiguous assignment of a temperature to a measured resistance is then not possible. Moreover, the prediction of the behaviour of the hot glow plug on the basis of the cold resistance is also largely unfeasible in known methods.
Accordingly, a way in which the surface temperature of a glow plug can be controlled more precisely is desirable.